Move curve check out of tls12_check_peer_sigalg.
The current check has two problems:
- It only runs on the server, where there isn't a curve list at all. This was a
mistake in https://boringssl-review.googlesource.com/1843 which flipped it
from client-only to server-only.
- It only runs in TLS 1.2, so one could bypass it by just negotiating TLS 1.1.
Upstream added it as part of their Suite B mode, which requires 1.2.
Move it elsewhere. Though we do not check the entire chain, leaving that to the
certificate verifier, signatures made by the leaf certificate are made by the
SSL/TLS stack, so it's reasonable to check the curve as part of checking
suitability of a leaf.
Change-Id: I7c12f2a32ba946a20e9ba6c70eff23bebcb60bb2
Reviewed-on: https://boringssl-review.googlesource.com/6414
Reviewed-by: Adam Langley <agl@google.com>